Conventional FC SANs. FC is a serial transport protocol that was developed for carrying other transport protocols. In conventional SANs, FC carries Small Computer System Interconnect (SCSI), which is a parallel protocol. In other words, parallel SCSI commands are encapsulated within FC frames and transported over FC links in FC SANs.
FIG. 1 illustrates an exemplary conventional SAN 100 which includes one or more hosts 102 connected to two Redundant Array of Independent Disks (RAID) controllers 104 over a network 106. The host side of the RAID controllers 104 is referred to as the “front end” 112. In conventional SANs 100, the RAID controllers 104 are connected to a plurality (e.g. 30 to 100) of drives in disk drive enclosures 108 and send and receive FC frames over a FC link 110. The disk drive enclosure side of the RAID controllers 104 is referred to as the “back end” 114. In conventional SANs 100, the disk drives within the disk drive enclosures are FC drives 118 that operate according to the SCSI protocol.
FC-ATA SANs. FC drives offer the best performance, but are expensive. Therefore, less expensive (but lower performance) Advanced Technology Attachment (ATA) drives of the type commonly used in desktop or notebook computers have been used in place of FC drives, or along with FC drives in what is referred to as tiered storage. The ATA drives may be Parallel Advanced Technology Attachment (PATA) or Serial Advanced Technology Attachment (SATA) drives. FIG. I illustrates a SAN in which one of the disk drive enclosures 108 contain PATA drives 120 rather than FC drives. PATA drives require a FC-to-PATA bridge 116, which is relatively expensive and effectively makes the PATA disk drives 120 appear as SCSI drives to the RAID controller 104. In other words, the RAID controllers 104 send FC encapsulated SCSI commands to the disk drive enclosures, and receive FC encapsulated SCSI commands from the disk drive enclosures, and the conversion between FC and PATA occurs in the bridge 116, transparent to the RAID controllers 104 and the rest of the SAN 100. Because PATA drives are different from FC drives in terms of interfaces, error recovery and discovery, FC-to-PATA bridges are designed to be specific to a particular type of PATA drive. As a consequence, every time a new PATA drive is developed, the FC-to-PATA bridge may require modification.
In disk drive technology, as well as in transport technology, there are speed and cable distance benefits to utilizing serial protocols rather than parallel protocols. SATA drives, the serial counterpart to PATA drives, are therefore now being contemplated as an upgrade to PATA. SATA was envisioned for consumer applications.
SAS-SATA SANs. FC, as described above, is a serial transport protocol that has historically been used for carrying the SCSI protocol in enterprise applications over large connectivity spaces. Serial Attached SCSI (SAS) is a relatively new serial protocol intended to replace parallel SCSI within an enterprise host or computer. Both FC and SAS use 8b10b encoding and similar ordered sets, and both are high performance and expensive. SAS includes several protocols. One such protocol is the Simple Management Protocol (SMP), a protocol for device-to-device management that enables each entity to communicate with other entities regarding management aspects.
To take advantage of lower cost SATA drives, SATA drives have been utilized alongside higher cost, higher performance SAS drives in SAS networks (a SAS network including the initiator, target, and any attached expander devices). As mentioned above, tiered storage is the concept of having different types of drives in the same network (e.g. some 73 GByte FC drives and some 200-500 GByte SATA drives), each for a different purpose. FIG. 2 illustrates a SAS SAN incorporating tiered storage, where SATA drives are utilized in addition to SAS drives. As illustrated in FIG. 2, within a host 200, a motherboard (MB) 202 includes a processor 204, an embedded SAS Input/Output Controller (IOC) 206, and a SAS expander 208 to provide multiple ports to the MB 202 and multiple connections to drives. Connected to the host 200 are SAS drives 210 and SATA drives 212 within the host 200. In addition, the host 200 is connected to enclosures 214 containing both SAS and SATA drives. To accommodate tiered storage, another protocol was developed within SAS, the SATA Tunneling Protocol (STP), which enables lower cost SATA drives to be employed in SAS systems.
Unlike FC, which is a loop technology where drives share a common infrastructure, SAS is a point-to-point technology. SAS employs a shared infrastructure with the ability to create a point-to-point connection between two devices through which data may be transferred without interruption. Similar to FC, SAS goes through a discovery process where the first SAS entity that is discovered is the SAS expander 208. The number of ports in the SAS expander 208 is also discovered. Each port is then discovered in turn by the initiator, and the device connected to each port is determined (e.g. a SAS device). For example, if a SAS discovery ordered set is sent to a SAS drive, the SAS drive returns an affirmative response indicating that it is a SAS drive. However, if the SAS ordered set is sent to a SATA drive, nothing is returned. Similarly, if a SATA discovery ordered set is sent to a SATA drive, the SATA drive returns an affirmative response, indicating that it is a SATA drive. From that point forward, the initiator communicates with the device as a SATA device.
In the simplified ladder diagram of FIG. 2 showing a half-duplex operation, SAS ordered sets are sent between the initiator 200 and the enclosure expander. The enclosure expander makes a connection between the initiator 200 and the correct target. Once the connection is created, SATA ordered sets 216 flow between a host or initiator 200 and a target 218. The SAS communications effectively build a point-to-point connection between the SAS IOC 206 and a target (e.g. SATA drive 212), and thereafter SATA ordered sets are passed across this connection that are natively understood by the SATA drive 212. Intermixed with the SATA ordered sets will be SATA File Information Structures (FISs) flowing from the initiator 200 to the target 218 (see reference character 220), and from the target 218 to the initiator 200 (see reference character 222) according to STP.
Because of the reliability, speed and cable distance benefits inherent in FC, and the lower cost of SATA drives, there is a need to utilize SATA drives in FC SANs that have historically utilized SCSI drives. Conventional solutions for utilizing SATA drives in FC SANs provide a conversion interface, or bridge, between the FC link and the SATA device. These conversion interfaces terminate all FC exchanges and initiate corresponding SATA exchanges at or near the targets. These bridging solutions require a bridge unit per SATA device or a bridge per SATA enclosure and as a result become a prohibitively expensive solution in a SAN environment. In addition, all error cases must be dealt with at or near the drive level. In the other direction, SATA exchanges are also terminated and FC exchanges are created and sent to the FC initiator. Because the FC to SATA translation is performed independently at each SATA drive or enclosure, there is no clean way of performing this conversion and the approach is prone to performance and interoperability issues. Error recovery in FC is also much different than SATA. The interface must now deal with the differences, which adds complexity and additional cost to the system.
Therefore, there is a need to be able to utilize SATA drives while preserving the FC infrastructure and FC transport to the greatest extent possible to minimize the changes needed to legacy FC SANs. There is a further need to move the translation and protocol handling into the RAID controllers, which is a much more cost effective solution because the RAID controllers can perform the protocol translation for a large number of drives.